Fullerenes are closed-cage molecules composed entirely of sp2-hybridized carbons, arranged in hexagons and pentagons. Fullerenes (e.g., C60) were first identified as closed spheroidal cages produced by condensation from vaporized carbon (Kroto et al., Nature, 1985, 318:162; and Krätschmer et al., Nature, 1990, 347:354). Fullerene tubes, being elongated tubular fullerenes, are produced in carbon deposits on the cathode in carbon arc methods of producing spheroidal fullerenes from vaporized carbon—although numerous other methods exist for making them. See Iijima, Nature, 1991, 354:56-58; Ebbesen et al., Nature, 1992, 358:220; and Ebbesen et al., Annual Review of Materials Science, 1994, 24:235-264. Such tubes are referred to herein as carbon nanotubes. Many of the carbon nanotubes made by these processes were multi-wall nanotubes (MWNTs), i.e., the carbon nanotubes resembled concentric cylinders having multiple walls or shells arranged in a manner analogous to that of Russian “nesting dolls.” Carbon nanotubes having up to seven walls have been described in the prior art (Ebbesen et al., Annual Review of Materials Science, 1994, 24:235-264; Iijima et al., Nature, 1991, 354:56-58).
Single-wall carbon nanotubes (SWNTs) were discovered in 1993 in soot produced in an arc discharge in the presence of transition metal catalysts (Iijima et al., Nature, 1993, 363:603-605). Such SWNTs, comprised of a single tube of carbon atoms, are the smallest of the carbon nanotubes. SWNTs can typically have lengths of up to several micrometers (millimeter-long nanotubes have been observed) and diameters of approximately 0.5 nm-10.0 nm (Saito et al., Physical Properties of Carbon Nanotubes, 1998, London: Imperial College Press; Sun et al., Nature, 2000, 403:384), although most have diameters of less than about 2 nm (Saito et al.). Diameters as small as 0.4 nm have been reported, but these were formed inside either MWNTs (Qin et al., Chem. Phys. Lett., 2001, 349:389-393) or zeolites (Wang et al., Nature, 2000, 408:50-51). SWNTs, and carbon nanotubes of all types have since been produced by other techniques which include chemical vapor deposition (CVD) techniques (Hafner et al., Chem. Phys. Lett., 1998, 296:195-202; Cheng et al., Chem. Phys. Lett., 1998, 289:602-610; Nikolaev et al., Chem. Phys. Lett., 1999, 313:91-97), laser ablation techniques (Thess et al., Science, 1996, 273:483-487), and flame synthesis (Vander Wal et al., J. Phys. Chem. B., 2001, 105(42): 10249-10256).
Since their discovery, there has been a great deal of interest in the functionalization (sometimes referred to as derivatization) of carbon nanotubes and, more particularly, in single-wall carbon nanotubes, to facilitate their manipulation, to enhance the solubility of such nanotubes, and to make the nanotubes more amenable to blend and composite formation. This is because single-wall carbon nanotubes are one of the more striking discoveries in the chemistry and materials genre in recent years. Such nanotubes posses tremendous strength, an extreme aspect ratio, and are excellent thermal and electrical conductors. A plethora of potential applications for nanotubes have been hypothesized, and some progress is being made towards commercial applications (Baughman et al., Science, 2002, 297:787-792). Accordingly, chemical modification of single-wall carbon nanotubes, as well as multi-wall carbon nanotubes, will be necessary for some applications. For instance, such applications may require grafting of moieties to the nanotubes: to allow assembly of modified nanotubes, such as single-wall carbon nanotubes, onto surfaces for electronics applications; to allow reaction with host matrices in polymer blends and composites; and to allow the presence of a variety of functional groups bound to the nanotubes, such as single-wall carbon nanotubes, for sensing applications. And once blended, some applications may benefit from the thermal removal of these chemical moieties, as described in PCT publication WO 02/060812 by Tour et al., filed Jan. 29, 2002.
While there have been many reports and review articles on the production and physical properties of carbon nanotubes, reports on chemical manipulation of nanotubes have been slow to emerge. There have been reports of functionalizing nanotube ends with carboxylic groups (Rao, et al., Chem. Commun., 1996, 1525-1526; Wong, et al., Nature, 1998, 394:52-55), and then further manipulation to tether them to gold particles via thiol linkages (Liu, et al., Science, 1998, 280:1253-1256). Haddon and co-workers (Chen, et al., Science, 1998, 282:95-98) have reported solvating single-wall carbon nanotubes by adding octadecylamine groups on the ends of the tubes and then purportedly adding dichlorocarbenes to the nanotube sidewall, albeit in relatively low quantities (˜2%).
Success at covalent sidewall derivatization of single-wall carbon nanotubes has been limited in scope, and the reactivity of the sidewalls has been compared to the reactivity of the basal plane of graphite (Aihara, J. Phys. Chem., 1994, 98:9773-9776). A viable route to direct sidewall functionalization of single-wall carbon nanotubes has been fluorination at elevated temperatures, which process was disclosed in a patent commonly assigned to the assignee of the present Application, U.S. Pat. No. 6,645,455, to Margrave et al. These functionalized nanotubes may either be de-fluorinated by treatment with hydrazine (or allowed to react with strong nucleophiles, such as alkyllithium reagents. See Mickelson et al., J. Phys. Chem. B, 1999, 103; 4318-4322; and Boul et al., Chem. Phys. Lett., 1999, 310:367-372, respectively. Although fluorinated nanotubes may well provide access to a variety of functionalized materials, the two-step protocol and functional group intolerance to organolithium reagents may render such processes incompatible with certain, ultimate uses of the carbon nanotubes. Other attempts at sidewall modification have been hampered by the presence of significant graphitic or amorphous carbon contaminants. See Chen, Y. et al., J. Mater Res. 1998, 13:2423-2431. For some reviews on sidewall functionalization, see Bahr et al., J. Mater. Chem., 2002, 12:1952; Banerjee et al., Chem. Eur. J., 2003, 9:1898; Holzinger et al., Angew. Chem. Int. Ed., 2001, 40(21):4002-4005; Khabashesku et al., Acc. Chem. Res., 2002, 35:1087-1095; and Dyke et al., J. Phys. Chem. A, 2005, 108:11151-11159. Within the literature concerning sidewall-functionalization of SWNTs, however, there is a wide discrepancy of solubility values between reports. This is due to explicable variations in filtration methods.
A more direct approach to high degrees of functionalization of nanotubes (i.e., a one step approach and one that is compatible with certain, ultimate uses of the nanotubes) has been developed using diazonium salts and was disclosed in a co-pending application commonly assigned to the assignee of the present Application. See PCT publication WO 02/060812 by Tour et al., filed Jan. 29, 2002. Using pre-synthesized diazonium salts, or generating the diazonium species in situ, reaction with such species has been shown to produce derivatized SWNTs having approximately 1 out of every 20 to 30 carbons in a nanotube bearing a functional moiety.
Because of the poor solubility of SWNTs in solvent media, such processes require extraordinary amounts of solvent for the dissolution and/or dispersion of the SWNTs (˜2 L/g coupled with sonication in most cases). See Bahr et al., Chem. Commun., 2000, 193-194. This problem of an inordinate amount of solvent makes covalent functionalization on the industrial scale economically infeasible. This has led to the development of solvent-free methods for the functionalization/derivatization of carbon nanotubes. See Dyke et al., J. Am. Chem. Soc., 2003, 125:1156-1157.
While the above-mentioned functionalization methods have enhanced the manipulability of carbon nanotubes (especially SWNTs), the use of functionalization/derivatization methods to produce novel composite materials and structures has not been fully explored. From the standpoint of the electronic/semiconductor industry, methods of anchoring and/or grafting carbon nanotubes to silicon or other surfaces would be highly advantageous, particularly where such anchoring and grafting is done without metal.